CHICAGO (Reuters) - Diabetes may lower the heart-protective benefits of high-density lipoprotein, or HDL, the so-called "good" cholesterol, but giving diabetics niacin, a drug that raises HDL levels, might restore the benefit, researchers said on Tuesday.

HDL lowers heart risks because it clears "bad" low-density lipoprotein, or LDL cholesterol, away from arteries and back to the liver, where it is passed out of the body. Several recent studies also suggest HDL protects arteries by promoting cell healing and repair.

But in people with diabetes, HDL may be less protective, researchers at the University Hospital Zurich and the Medical School of Hannover in Germany reported in the journal Circulation.

The team compared the vessel-protecting action of HDL samples from 10 healthy adults with that of 33 patients who had type 2 diabetes and metabolic syndrome, a condition that includes having low levels of HDL. All of the people with diabetes were taking drugs called statins to lower their bad cholesterol.

In the lab, the team found that the protective benefits on blood vessels were "substantially impaired" in HDL taken from the diabetic patients compared with that taken from healthy people.

Next, they divided up the diabetics and gave half a placebo and treated half with extended-release niacin, a drug that raises HDL levels in the blood.

After three months, patients who got the niacin had increased HDL levels, and markedly improved protective functions of HDL in laboratory testing as well as improved vascular function.

Because it was a small study, the team said more research is needed to see if diabetics will benefit from getting extended-release niacin, such as Niaspan made by Abbott Laboratories Inc.

In the United States, about 11 percent of adults have diabetes. Most have type 2 diabetes, the kind closely linked to obesity.